(a) Field of the Invention
This invention relates to metallised fabrics, and in particular is concerned with a so-called microporous metallised fabric suitable for use where thermal insulating properties are required. The invention also relates to a method of manufacturing such a fabric.
(b) Description of the Prior Art
It is known to metallise a least one surface of a fabric in order to enhance the thermal insulating properties of that fabric. For the case of a porous fabric, provided that the metallising is performed in accordance with known procedures, the metallising does not significantly affect the porous nature of the fabric, nor does the metallising significantly reduce the durability of the fabric; moreover, the metallising often increases the flexibility of the fabric. As a result, metallised fabrics of this kind have been used in the manufacture of apparel intended to be worn in extreme climatic conditions, and also in the manufacture of articles required to have excellent thermal-insulating characteristics coupled with light weight, such as blankets and sleeping bags. More recently, such metallised fabrics have been used in the manufacture of blinds for the screening of glass-houses: by drawing out a blind of such fabric over and around the crop-growing area of a glass-house when the external ambient temperature is below that within the glass-house, the heat loss from the glass-house can greatly be reduced, leading to much lower heating costs.
The substrate fabric for use in the manufacture of a metallised fabric of the kind described above may be woven from natural fibres, such as cotton fibres, or may be woven from blended natural and synthetic fibres or even just from synthetic fibres. More commonly however the substrate fabric is manufactured from continuous, relatively fine fibres of a synthetic resin (polymer) such as a polyethylene or a polyester, which fibres are spread with a random orientation into a thin layer, and then united by the application of heat and pressure; such a manufacturing method can be performed in manner known per se so that the finished fabric has a microporous structure. A synthetic microporous fabric of this kind is sold by E. I. du Pont de Nemours, Inc., under the Trade Mark TYVEK, Style 1621C or 1622E.
A substrate fabric of the kind just described above may be metallised, conventionally with aluminum, by means of a vacuum deposition technique. This metal has excellent thermal reflective properties which greatly enhance the thermal insulative characteristic of the finished metallised fabric, and also aluminum particularly lends itself to deposition in this way. Moreover, it is possible to deposit a sufficiently small amount of aluminum so as not significantly to affect the porous nature of the fabric whilst still imparting to the fabric the required heat reflective properties. The porous nature of the metallised fabric is most important for many of the uses of the fabric, where the fabric must be able to "breathe"--that is to say, moisture laden air may pass through the fabric.
As mentioned above, metallised fabrics of the just-described kind have been used for the manufacture of blinds for the thermal insulation of glass-houses. When metallised, the aluminum metallising is directed outwardly, and the blind relies on the so called `emissivity` characteristic of the metallised fabric--that is to say, the ability of the fabric to radiate heat inwardly of the glass-house from the non-metallised surface. However, experience has shown that a glass-house blind made of this material may have a very limited life, in that the aluminum metallising relatively quickly starts detaching from the substrate fabric. Though the rate of detachment might be greatest where the fabric is subjected to the greatest mechanical stresses--for instance by friction or abrasion on fixed components, or by flexing or crumpling of the fabric--nevertheless sometimes the metallising does detach even where the mechanical stresses are quite small. The reasons for this are not fully understood but are thought to be connected with the high humidity environment which often prevails in a glass-house or possibly connected with the precise chemical composition of such liquid as may contact or collect on the top surface of the glass-house blind. A somewhat similar problem has been noted when metallised fabrics are used in the presence of moisture, for instance in the case of clothing.
In an attempt to prolong the life of a metallised fabric subjected to a hostile environment, recently various attempts have been made to enhance the adhesion of the aluminum metallising to the substrate fabric, but up to the present time these attempts have met with little or no success. Increasing the thickness of the metallising can reduce the flexibility of the fabric, leading to yet more rapid detachment of the metallising if the fabric is crushed or crumpled, and in any event an increased metallising thickness tends to block the pores of the fabric. On the other hand, a protective post-treatment such as the application of a lacquer, varnish or other siccative paint-like coating also tends to block the pores either completely or to an unacceptable extent, if that post-treatment is to have any effect.